Magnetorheological fluid (MRF) damper/strut force characteristics have been controlled by varying electrical current to an electromagnet(s), which affect the magnetorheological (MR) fluid's apparent viscosity. Increasing the magnetic field can lead to increased apparent and effective viscosity of an MRF. These types of controllable damping devices can change its force response in milliseconds through the magnetic field dependent material properties of an MRF.
Magnetorheological fluids often consist of micron-size ferrous particles suspended in a carrier fluid such as 1-10 micron sized spherical carbonyl iron particles suspended in silicone oil. The material's rheological change occurs when a magnetic field causes the ferrous particles to polarize and attract, thus, increasing the apparent viscosity of the MRF. As the magnetic field strength increases, the dipole moment created within the suspended ferrous particles increases, thus increasing the attraction between the suspended particles. As stronger attraction forces are created with increasing external magnetic field strength, the suspended particles form structured columns that increases the apparent viscosity of the MRF, resulting in higher resistance to fluid flow and decreased fluid compressibility.
Background references include: U.S. Pat. No. 5,277,281 to Carlson et al, entitled Magnetorheological Fluid Dampers; U.S. Patent No. 5,316,272, to Davis, entitled Liquid Spring Vehicular Suspension System and Associated Control Apparatus; U.S. Pat. No. 6,131,709, to Jolly et al, entitled Adjustable Valve and Vibration Damper Utilizing Same; U.S. Pat. No. 6,151,930, to Carlson, entitled Washing Machine Having a Controllable Field Response Damper; U.S. Pat. No. 6,497,309, to Lisenker, entitled Magnetorheological Damper With An External Coil; U.S. Pat. No. 6,510,929, to Gordaninejad, entitled Controllable Magnetorheological Fluid Damper; U.S. Pat. No. 6,547,044, to Lisenker, entitled Magnetorheological Damper With Ferromagnetic Housing Insert; and Published International Application No. WO 99/27273, by Jolly et al, entitled Adjustable Valve and Vibration Dampers.